This invention is specially directed to a negative photoresist derived from a film-forming photosensitized ("PS") polymer composition in which a ring-opened tetracyclic norbornene ("NB") such as tetracyclododecene ("TD"), optionally substituted with non-polar groups, predominates (hence the term "polynorbornene"). The photoresist meets a particular critical criterion in the manufacture of integrated circuits, namely high resolution for high performance.
A resist is a radiation-sensitive material which is coated on a substrate and exposed to the radiation through a pattern mask, the exposure causing differential solubility between the exposed and unexposed regions which upon solvent development, will reproduce an image of the mask as a thin pattern of resist which is left on the substrate. In a negative photoresist formed from a PS polymer composition, the exposed regions are photocrosslinked while the unexposed regions are not. Subsequent processing (e.g. etching, doping, ion milling, and the like) of the resist-covered substrate affects only the open areas while the areas protected by the pattern remain unprocessed, so that after removal of the resist, the substrate has processed and unprocessed regions.
When a thin film of this PS polymer composition is exposed to ultraviolet ("u-v") light having a wavelength of from about 220-450 nanometers in the contact mode, so as to form a pattern of lines and spaces, the resolution in the pattern is in the range from about 1 micron to about 2 microns, and preferably 1 microns or better, when the thickness of the film is 1 micron before the film is exposed.
Numerous copolymers are known which contain a double bond capable of reacting with a bis-azide crosslinking agent (referred to herein as an "azide-reactive double bond") so as to provide a nitrene radical to which is attributable the photosenstivity of the composition.
Among such polymers containing an azide-reactive double bond are the ring-opened polymers or norbornene ("NB" for brevity) and its derivatives. PS compositions of particular ring-opened NB derivatives are disclosed in U.S. Pat. No. 4,106,943 (the '943 patent). Like the prior art 1,3-diolefin polymers used for negative photoresists, ring-opened NB polymers are also hydrocarbons with olefinic unsaturation, and as in PS compositions generally, it is essential, if they are to perform superbly, that they have special physical and chemical properties which are attributable to the hydrocarbon backbone. What appear to be small differences in the structure of the backbone result in very large differences in properties. At present there is no realistic or reliable way in which one might predict what changes in the structure of the polymeric backbone might result in particular photoresist properties. Therefore, the photoresist properties of a likely PS composition are investigated after the type of specific configuration of a repeating unit, and the type of bond formation is a polymer formed is duly established.
Though the '943 patent contains a description in Example 1 of how the ring-opened polymer is prepared, I have been unable to have the polymer of methyl-5-norbornene-2-carboxylate made as taught. Though the carboxylate polymer, and other polymers of norbornenes substituted with polar substituents may be made, photosensitive compositions based on such polymers do not provide a resolution in the range from 1 to 2 microns when a film 1 micron thick is exposed to u-v light in the range from 220-450 nanometers.
Further, the carboxylate polymer made was not soluble in a substantially aliphatic hydrocarbon solvent, most particularly it is essentially insoluble in Stoddard solvent, and therefore no developable in it after exposure to adiation to form an image ("image-wise exposure") on the PS film. By "substantially aliphatic hydrocarbon solvent" I refer to an aliphatic solvent obtained as a petroleum distillate such as Stoddard solvent which is a refined naphtha not less than 50% boiling over 177.degree. C., 90% over 190.degree. C., and the end point higher than 210.degree. C. (see U.S. Bureau of Standards and ASTM D-484-52). Typically, Stoddard solvents contain no more than 10% by weight (wt), and usually less than 5% by wt of aromatic solvents.
By "soluble" I refer to the ability of the uncrosslinked PS polymer composition to form a gel-free solution (that is essentially 100% of the polymer is dissolved). By "developable" I refer to the ability of the developer ("solvent") to selectively dissolved the uncrosslinked polymer without substantially affecting the crosslinking polymer, that is, without evidence of contour deterioration by swelling. It is therefore implicit that the requirements of the solvent forming a solution from which the film is cast should be different from those of the developer.
Typically the casting solvent should have a boiling point (b pt) in the range from abut 110.degree. C. to about 140.degree. C. A lower b pt than specified leads to bubble formation or condensation of atmospheric moisture on the film since the film is deposited by high-speed spinning on a substrate under ambient room conditions. A higher b pt than specified leads to residual solvent in the film, softening the film, denigrating the resolution, and potentially leading to defects in the mask exposed in the contact mode. The requirements of the developer are that it dissolves the uncrosslinked film slowly, without substantially swelling the crosslinked film, therefore a higher b pt solvent than one in the specified range for casting solvent, is preferred. Implicity, a PS composition which is high in solubility in organic solvents or water, though excellent in sensitivity and stability, will nevertheless have poor resolution, typically from about 3 to 6 microns.
Essential criteria for an acceptable negative photoresist composition are adequate adhesion to a substrate upon which the film is deposited, and adequate photoresist solution stability. It is evident that the adhesion must be sufficient to secure the film to the substrate during subsequent etching. Yet, because the films of the '943 patent are not peeled off or torn off even by a strong shock at the time of development, high resolving power is ascribed to such films. Clearly, resolution is unrelated to adhesion of the film. If the film peels away during processing there can be no measurement or resolution; if it does not, the resolution may be poor or excellent, but not because the film adheres to the substrate whether such adherence is strong or weak.
Photosensitivity of a polymer compostion is unrelated to the resolution obtainable with it. A polymer with excellent photosensitivity, upon being photocrosslinked, may be easily swollen during development of the image resulting in unacceptably low resolution for an integrated circuit with a high density of devices. Sensivity, more precisely photospeed, determines the lenght of the exposure required to product an image with a preselected intensity of u-v light, not the resolution which may be realized. A typical exposure energy of 10-30 mJoule/cm.sup.2 is characteristic for a relatively sensitive film such as one based on cyclized polyisoprene (e.g., Kodak 747, Waycoat IC); doubling or trebling the time for a relatively less sensitive film is not necessarily a detriment if upon development the film yields a higher resolution.
Since the requirement of lack of swelling to provide the critical 1 micron resolution is so stringent, it is evident that there is no indication that any single photoresist based on the '943 compositions would meet this requrement.
It is well known that the theoretical resolution of an optical system as defined for the contact mode is a function of the wavelength of the radiation to which the film is exposed, and, the thickness of the film. The wavelength is preselected in the u-v range from about 220-450 namometers. Since the minimum film thickness is about 0.7 micron to avoid pinholing, the best theoretical resolution is in the range from about 0.4 to about 0.6 micron, depending upon the wavelength in this range.
Commercial negative photoresists have a resolution in the range from about 2.5 to 3.0 microns in a 1 micron thick film exposed in the contact mode. This relatively poor resolution is attributable to the swelling of the photocrosslinked composition. When an integrated circuit is minaturized to incorporate a larger number of devices on the same size of chip, the resolution of the photoresist composition becomes critical.
There is no teaching in the prior art that the result effectiveness of a NB polymer which forms a critically high resolution negative photoresist not heretofore obtained, was predicated upon the necessary presence of a major molar amount of a ring-opened unsubstituted or non-polar group substituted tetracyclic NB, optionally with a minor amount of another NB monomer in a polycyclic norbornene (hence "PolyNB") polymer. By "high resolution" I refer to a resolution of from 1 to 2 microns, more preferably about 1 microns. By "another NB monomer" I refer to a NB having fewer than four fused rings, which NB provides a repeating unit containing a ring-opened (bicyclo(2.2.1)-hept-2-ene) namely `norbornene`, which may have acyclic or cyclic (spiro) substituents such as alkylnorbornene, cycloalkylnorbornene, phenylnorbornene ("PNB"), and the like; or, a repeating unit containing a ring-opened dicyclopentadiene ("DCPD") which may be substituted.
Nor is there any suggestion that an uncrosslinked NB polymer having the particular structure of a PolyNB polymer referred to hereinabove would have a T.sub.g above 150.degree. C. A T.sub.g above this temperature happens to produce the desired critical performance as to resolution if exposed PS film cast with the polymer was developable in a substantially aliphatic hydrocarbon solvent. A high performance negative resist has been long sought after in the art (see Solid State Technology, by Elliott, David J., July 1983, p 139).
Cyclopentadine, a readily available byproduct from the production of ethylene, is a precursor of NB monomers, but these monomers have been only limited application in the formation of photoresists from Nb polymers because of their relatively poor resolution and high cost in comparison to polyisoprene-based PS compositions.
NB and substituted NBs may be produced by a Diels-Alder reaction of cyclopentadiene ("CPD") or DCPD with selected cyclic or acyclic olefins including 1-olefins, 2-olefins, 3-olefins and cyclopentene. U.S. Pat. No. 3,074,918 teaches the polymerization of cyclic olefins having at least one unsubstituted ring double bond and not more than one double bond in each ring such as DCPD, dihydroDCPD, NB, or substituted NBs. U.S. Pat. No. 3,546,183 describes elastomer polymers or polymers which may be used as bases for elastomer compositions, having in their structural formula units of alkyl-5-bicyclo(2.2.1-hept-2-ene or alkoxy-5-bicyclo(2.2.1) hept-2-ene. U.S. Pat. No. 4,178,424 describes the preparation of copolymers of tetracyclododecenes ("TD") and DCPD which are fused-ring monomers which may have ring substituents, and also substituted monocyclo NBs which may have spiro and other substiuents. In the foregoing and other polymerizations based on the ring-opening of a NB or substituted NB monomer, an alpha-olefin such as 1-hexene is used as a molecular weight regulator, the alpha-olefin functioning as a chain terminator. Other U.S. patents disclosing NB-type polymers include U.S. Pat. Nos. 2,721,189; 2,831,037; 2,932,630; 3,330,815; 3,367,924; 3,467,633; 3,836,593; 3,879,343; 4,020,021; and 4,136,249; and, the disclosures of each of the foregoing are incorporated by reference thereto as if fully set forth herein.